Split-phase servomotor control



5. BERTRAM SPLIT-PHASE SERVOIOTOR CONTROL Filed Oct. 3} 1944 Nov. 19, 1946.

NVENTOR BERT/M n Mfi Y a H y mg m In M "1w .m I PM H mg WM "M0 T ll diate and 90 Patented Nov. 19, '1946 -FtlCE 2,411,357 SPLIT-PHASE SERVOMOTOR CQNTRGL Sidney Bertram, San

the "United States G by the Secretary of Diego, Callil, assignor to overnment, as represented the Navy Application October 3, 1944, Serial No. 556,989

:11 Claims. (oi. 172-278) This invention relates to a servo-slave mechanlsm, and particularly to a circuit for regulating the rotating fields of polyphase or split-phase induction motors.

There are many instances where it is most desirable and convenient to provide a follower means, where it is, at the same time, impossible to load th means to be followed. It is one of the objects of the invention to provide a circuit, including a two-phase or split-phase. induction motor, which will drive the following means in such manner that it follows the movement of the device whose movement is tied.

It is well known in the art that the rotating field produced by the stator windings in a twophase induction motor is the resultantoi two independent fields, which are produced by separate windings. It is also familiar to those skilled in the art that th torque produced in such a motor is dependent upon the relative magnitudes of these fields and their electrical phase displacement with respect to one another. Assuming constant magnitude, maximum torque is pro duced when the two fields are as electrical degrees out of phase with one another; and minimum, or zero, torque is produced when the two ilelds are in phase. Phase differences intermeproduce intermediate torques. "When one phase of the stator winding leads the second phase, rotation in'a given direction results. When the phase relationship is reversed, so that the first phase lags the second phase, rotation in the other direction is produced. Generally, then,

the invention comprises means for governing this phase relationship in a novel manner such that the motor drives the follower means in one direction or the other, or when no phase difference exists, not at all.

in its simplest form, the invention comprises a photo cell connected to fire a trigger-acting or discharge tube of the Thyratron type when the means to h followed causes a change in the light intensity which governs the photo-cell. The cathode-anode path or the discharge tube is connected in series with the primary or a transformer whose secondary is placed across one phase of the stator windings of the motor, so that when fired, the discharge tube takes control of the voltage supplied to that phase of the stator and supplies such voltage in opposite phase to that normally supplied through a separate inductance when the discharge tub is not firing.

The device as described has very broad appllcation and may he used for a variety of purposes,

to be repeated or ampllwinding of the stator.

among which may be listed the following: (1) training devices which may be to follow a rotating member such as a compass card, (2) industrial applications such as the maintenance of constant tension on moving belts, (3) operation of doors, drinking fountains, safety devices, etc, where interruption of the light beam may he made to result in a shifting or sliding rather than a rotating held, and. (4) any photo-cell actuated servo-mechanism In the drawing:

He. 1 is a schematic diagram of the invention.

Fig. 2 is a series of curves laid out along a common abscissa and showing voltages in various I portions of the circuit.

In Fig. 1 is shown a conventional two-phase motor in which the four terminals represent those connected to the windings of the twophase Terminals l, l on the motor are fed through a phase-shifting capacitor l and th other winding is normally led, through an inductance S, at terminal 2. Alternative 1y. a conventional split-phas motor might be used in place or the two-phase unit, in which case the phase-shifting capacitor would he eliminated. A gas tube which may he of the conventional RCA 2056 'Ihyratron type, is connected in series with the primary of transformer l across the supply terminals 3, The secondcry of transformer l is also connected across the second phase of the stator at terminals 3. a

The gas tub is controlled a g'zhoto-cell 8, which may he of the RCA Q2? trace, connected to its grid. Grid bias for the tube 6 is supplied through resistor potentiometer it, while the shield grid is tied to the thesis. This bias is supplied across potentloninh l 9 by means of a transformer ii, whose gorl vy is connected across the voltage supply 1 ilnals El, El. Al though not necessary to the i tioning of the invention, a capacitor l2 may series with-and on the high sid or potentiometer ill for a purpose to be desert In operation, the voltage sup heel terminals 9, i, being fed through capacitor: leads the applied voltage by substantially Qt". The other phase winding of the motor a a terminals 2, 2 through intluc succtube 6, when is not circuits the primary oi transfo these conditions. the inducta .ce 0nd phase or" the motor with a voltage approximately 93 out of phase In that supplied at terminals 3, i. This is tru since when the tube connected in a rees? 6 is not firing and the primary of transformer i is effectively open circuited, the secondary oi the transformer, although connected across the second phase of the motor, has very little efiect, its impedance being arranged to be very high. The motor then runs in a direction determined by the (in effect) direct connection of the second phase of the motor across the supply voltage. This is explained by the fact that the motor winding across terminals 2, 2 is, of itself, highly inductive and the effect of inductance 6 is to cause only a lowering of the applied voltage rather than shifting its phase.

The photo-cell 8 is so arranged that when light falls upon it, the current flow causes a substantial change in the grid bias on gas tube ti, so that when its p ate becomes positive with respect to the cathode, the tube fires and the primary of transformer l is efiectively connected directly across the supply voltage during positive excursions. of thevoltage. The resulting D. C. (rectified) component voltage appears across the secondary of transformer l as a large line frequency component, plus harmonics which have no effect on the resulting motor torque. Transformer 1 is so phased that when tube 6 is firing its output is in the opposite phase (180) to that supplied through inductance 5. Since the impedance of transformer l is very materially reduced while.

' operation tube 6 is firing, the motor is supplied witha voltage of such reversed phase, and the inductance 5 appears as a relatively high impedance. The application of this voltage, reversed in phase, to terminals 2, 2 of the motor, causes a reversal in the direction of its rotation;

The actual operation may be better understood by studying the curves in Fig. 2. Curve A represents thevoltage which is supplied to the terminals 3, 3 by the external supply. Curve B represents the voltage applied to terminals i, l regard less of operation of the photo-cell and gas tube, and it will be noted that it is 90 out of phase with'the supply voltage of curve A, due to the action of capacitor 6.

Curve C represents the voltage which is applied to terminals 2, 2 when tube ii is not firing. In this case, since inductance 5, as has been stated, has little effect upon the phase of the applied voltage, the curve shows that the voltage supplied t terminals 2, 2 is of the same phase as the supply voltage of curve A, but reduced in magnitude. Under these conditions, the voltage supplied to terminals 2, 2 (curve C) is 90 out of phase with that supplied to terminals i, i (curve B) and maximum torque is thus produced with the appropriate sign. Curve D shows the voltageappearing across the primary of transformer 7 when tube 6 is not firing. This voltage is imduced therein by the action of the secondary connected to terminals 2, 2. When tube 6 fires, the

' rectified voltage across the primary of transformer l is as illustrated in curve E and is seen to consist of a series of positive pulses. Spe cifically, the pulses of greater amplitude represent the voltage across the primary of transformer l during those half-cycles when tube 6 fires. The pulses of smaller amplitude represent the voltage during the alternate half-cycles and therefore are the same as the positive half-cycle pulses shown in curve D. Under these conditions, the voltage induced in the secondary of transformer l is shown by curve F and is opposite in phase to the voltage in the primary (curve E),

but with its line frequency component substantially 90 out of phase with that supplied to terminals 2, 2 (curve C). Curve F shows a large component of phase reversed to that in curve C being applied to terminals 2, 2 of the motor. However,

' of that induced in the secondary of the trans-= former l (curve E) and that supplied by reactance 5, and is illustrated in curve F. It should, of course, be noted that this latter component is of the same phase as that supplied by such reactance to terminals 2, 2 when the tube is not firing (curve C), but that its magnitude has been considerably reduced because, under firing conditions, reactance 5 is a relatively high impedance. In any event, the resultant line fre quency component applied to terminals 2, 2 when the tube is firing (curve F) is opposite (180) in phase to that applied when it is not firing (curve C). 0f even more importance is the fact that the line frequency component of the voltage thus applied to terminals 2, 2 under these conditions is also 90 out of phase with that applied at terminals I, l (curve B), and thus again maximum torque isproduced. However,- it is to be noted that the torque thus produced is in opposite direction to that produced when terminals 2, 2 are supplied through inductance 5 (curve C).

Somewhere between these limiting conditions,

of the device produces no torque because the resultant voltage applied to terminals 2, 2 is equal to zero. In this case, the component each cycle.

supplied from inductance 5 exactly cancels that supplied from the-secondary of transformer I, and the motor comes to rest.

The curves in Fig. 2, although generally illustrating the respective voltages, are not intended to be as accurate as those which might be obtained froman oscillograph. However, they are intended to and do illustrate the operation of the invention, for which purpose highly accurate plots, including the various harmonics, are unnecessary.

Although the above description includes the principal features ofthe invention, its usefulness may be considerably increased by an auxiliary capacitor i2. This capacitor, included in the grid circuit of gas tube 6, adds an A. C. voltage component which leads the plate voltage by substantially The use of such a capacitor results in the firing of the gas tube 6 earlier in the positive portion of the cycle, as the photo-cell 8 becomes increasingly conductive. Hence, the average length of time during which the tube is firing is made to depend upon the action of the photo-cell; and the voltage supplied to motor terminals 2, 2 is caused to shift in phase gradually rather than abruptly. Stated another way, there is a gradual shift in phase from a condition where motor terminals 2, 2 are supplied solely from inductance 5 (as in curve C), to a condition where the gas tube takes over control due to its firing for substantially one-half of At one point between these limits, i. e., for some particular intensity of light falling on the photo-cell 8, the resulting voltage applied to terminals 2, 2 will be of greatly reduced magnitude and in phase with that supplied to tenninals l, i. At this point, the motor will have no torque and will remain stationary. By varying the light falling on the photo-cell, the circuit is thus enabled to drive the motor in either direction, or to bring it to a full stop, with the torque roughly proportional to the change in intensity.

a,411,ss7

It should also be noted that for most efllcient operation, the supply voltage should be considerably larger than is normally required by the motor. This will allow the choice of inductance 5 with animpedance large compared to that of the motor when the tube is fired. This provides an additional advantage in thatthe gas tube may be operated at higher voltages, and

correspondingly lower currents are required for operation. This practice is necessarily limited by the con'ditionwherean exceedingly high volt-' age might cause the tube to flash over in a manner not consistent with the determining grid w voltage. Obviously the voltage and current must be maintained within the limits of the tube rating.

There is thus provided an eflective and simple servo-slave mechanism which curately repeating] or amplifying any desired motion.

As was briefly mentioned above. any element which exhibits mechanical distortion due to externa1 forces can easily be arranged to control the light falling on the photo-cell, which in turn enables the motor to realign such forces and again bring them into the desired relationship.

The applications discussed herein are in no -way intended to illustrate all of the uses to which the device may be put. They are representative only and many more will immediately occur to those skilled in the art.

I claim:

1. A servo mechanism comprising a source of alternating voltage, an armature, means connected across said source for producing a first magnetic field, an impedance, second means connected in series with said impedance across said source for producing. a second magnetic field is capable of acv ply a voltage eflectively 130 out of phase with a said source,

3. In combination with mechanism as described in claim 1, a capacitor connected in series with said first mentioned means.

4. In combination with mechanism as described in claim 1, means for supplying grid voltage to said tube with a phase substantially .diilerentfrom that supplied at saidsource.

5. A servo mechanism comprising a source of alternating voltage, a split-phase motor, one

winding of said motor being connected across an impedance connected in series with a second winding of said motor across said source, a grid-controlled gas tube, control means connected to the grid of said tube, a transformer, said tube and one winding of said transformer being connected in series across said source, the other winding 0! said trasformer being connected across said second winding of said motor to supthe voltage from said' source when said tube fires.

6. A mechanism described in claim 5 in which said control means comprises a photo-cell.

7. In a mechanism as described in claim 5, means for supplying rid voltage to said tube with a phase substantially diilerent from that supplied at said source. I

8. A servo mechanism comprising a source of alternating voltage, a two-phase motor. one phase of said motor being connected across said source, an impedance, said impedance being connected in series with the second phase of said motor across said source, a grid-controlled gas tube, control means connected to the grid of displaced with respect to said first magnetic field,

both oi! said ilelds acting on said armature, -a. vapor-electric discharge tube, control means coupled to said tube, a transformer, said tube and one winding of said transformer being connected in series across said source, the other winding of said transformer being connected across said second means to supply a voltage which is phase-displaced with respect to the voltage from said source when said tube fires.

2. A mechanism as described in claim 1 in which said control means comprises a photocell. 4

said tube, a transformer, ing 01' said transformer said tube and one windsupplied at said source.

SIDNEY BERI'RAM.

being connected in seriesv .across said source, the other winding 01' said 

